Heart disease remains the number one cause of death in the United States. It is estimated that as many people die from heart disease as from all other causes combined. For many cardiac patients, the cause of death is ventricular arrhythmia in the form of a sudden ventricular tachycardia or ventricular fibrillation that often occurs with little warning.
Ventricular tachycardia is a fairly regular cardiac rhythm disturbance originating in the ventricles which results in a ventricular rate that is too fast to effectively pump blood. Ventricular fibrillation is a chaotic rhythm that is incapable of pumping any blood. In both circumstances, if the blood flow is not reestablished within about three minutes the victim will begin to suffer irreversible brain damage and will most likely succumb within six minutes from the onset of the ventricular arrhythmia.
A substantial number of patients who suffer a bout of ventricular tachycardia or fibrillation survive, only to eventually succumb at a later time to a recurrence of their rhythm disturbance. It is for the benefit of these patients that implantable cardiac defibrillators (ICD) are being developed and refined.
In general, these devices accomplish defibrillation of the heart by delivering an electrical countershock to the myocardium via implanted electrode leads of sufficient strength to terminate the abnormal rhythm. In effect, the ICD simultaneously depolarizes the heart breaking the abnormal rhythm cycle thereby providing an opportunity for the patient's underlying normal rhythm to reestablish. The units are implanted within the patient and connected to the heart through various electrodes to provide continuous monitoring and immediate countershocking when a rhythm disturbance is detected. Because the devices must be small enough for convenient implantation, ICD's are limited in their ability to store electrical energy.
It is important to reliably deliver a sufficiently strong electrical shock to insure defibrillation in response to each occurrence of fibrillation. One method might be to use a set amount of energy known to defibrillate each time. However, this approach is an inefficient use of the limited stored electrical energy and will unnecessarily limit the useful life of a device equipped with this method. It is well known in the art that the energy required to effectively defibrillate a human heart varies with the implanted lead configuration and placement as well as the individual heart's responsiveness to the electrical countershock. To maximize efficiency and still retain integrity to effect defibrillation, it is necessary to determine as accurately as possible the minimal energy level necessary to defibrillate the patient's heart, i.e., the defibrillation threshold (DFT) after the device and the leads are implanted.
One known method of determining the DFT energy of an implantable system is to induce fibrillation with an electrical shock. The heart is then defibrillated through the implanted defibrillation leads. Initially, defibrillation is attempted at a relatively high energy level. If successful in defibrillating the heart, fibrillation is reinitiated and the defibrillation is attempted at a lower energy level. This process is repeated with successively lower defibrillation countershocks until the shock does defibrillate the heart. In this way, a DFT is established and the defibrillation energy level for the ICD is then set at an estimate of the lowest level that can reliably achieve defibrillation given the safety margin above. The significant disadvantage to this method is the necessity to repeatedly fibrillate and then defibrillate the patient's heart to determine the DFT.
Another method of determining defibrillation thresholds is disclosed in U.S. Pat. No. 5,105,809, issued Apr. 21, 1992. The method disclosed begins by applying an initial electrical shock to the patient's heart during a period of vulnerability. The period of vulnerability usually occurs contemporaneously with the T-wave of a standard electrocardiogram (ECG). The energy level of the initial shock is sufficiently high so as not to cause fibrillation. Assuming this initial shock fails to induce fibrillation, a second shock of less magnitude is delivered during a subsequent vulnerability period. The process is repeated with successive shocks of lesser magnitudes until fibrillation is induced. When fibrillation finally occurs, the energy of the preceding shock that did not cause fibrillation is deemed to be the energy level required to defibrillate for that particular lead configuration.
Unfortunately, the vulnerability period on which this technique relies differs from patient to patient and is not necessarily contemporaneous with the T-wave. As a result, this method is susceptible to inefficiency because the time period during the T-wave at which the testing is undertaken may have a substantially different vulnerability than at other points during the T-wave. This method substantially misses the most vulnerable period during the T-wave. This discrepancy will not be appreciated at the time of implantation and therefore substantially underestimates the defibrillation threshold setting.
Another method for establishing a defibrillation threshold is disclosed in U.S. Pat. No. 5,346,506, issued Sep. 13, 1994. The method disclosed relies on research demonstrating that the 50% probability of successful defibrillation can be closely approximated by determining the 50% probability of reaching the upper limit of vulnerability (ULV). See "Relation Between Upper Limit of Vulnerability and Defibrillation Threshold in Humans", Chen eta., Circulation vol. 88, no. 1, 186-92 (July 1993). The electrical energy is applied to the heart through epicardial patches at a predetermined limited period of time centered on the mid-upslope of the T-wave. The disclosure argues that by not having to scan the entire T-wave with shocks the total number of shocks is reduced. The disadvantage to this disclosure is that the energy level for the first application is estimated beforehand. The number of shocks required to determine the DFT is reduced only if the estimated 50% probability of reaching the upper limit of vulnerability is quite accurate.
Each of these methods necessitate the use of multiple electrical energy shocks to be delivered from an implantable device of limited storage capacity. Each test shock depletes energy from the device thereby decreasing the expected useful life-span of the device. Additionally, these repeated shocks and episodes of fibrillation take a significant toll on the patient. Considerable time must be spent between test cycles in order to provide the patient's heart time to recover from the previous round of shocks. Because the patient is typically under general anesthesia, then delays in determining the appropriate DFT increase the patient's anesthesia risk. Accordingly, it would be desirable to provide an ICD that is convenient to implant and reliable to use, and that can be used to quickly determine the defibrillation threshold using the least number of test shocks possible and minimizing the patient's risk to only one episode of fibrillation.